Emission regulations pertaining to engine exhaust emissions are increasingly become more restrictive throughout the world, including, for example, restrictions on emission of particulates and NO.sub.x. Tailoring the injection rate of fuel to a combustion chamber, as well as quantity and timing of such fuel injection, is one way in which to meet such emission regulations. At different engine operating conditions it may be necessary to utilize different injection rate waveforms in order to achieve optimum engine operation and emissions control.
In the past hydraulically-actuated electronically-controlled fuel injection systems have included some mechanical limitations on the variety of achievable injection rate waveform types. In some systems the injectors utilized have also been somewhat limited as to the control current waveform which could be utilized. Resulting problems included injecting fuel too rapidly within a given injection event and allowing fuel to be injected beyond a desired stopping point. Such problems can adversely affect emission outputs and fuel economy.
In a system in which different injection rate waveforms are achievable, it has been determined that transfer from one type of waveform to another type of waveform may need to be controlled under certain circumstances in order to prevent excessive noise, as well as to prevent repeated transition back and forth between two different waveform types.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.